The present invention relates to a thread arrester for weft feeders for air-jet looms.
It is known that weft feeders are devices that accumulate a reserve of thread in the form of turns wound around a fixed reel or drum and feed the loom by unwinding the turns in an amount equal to the length L of thread required by the loom at each beat, said length being equal to the width of the fabric being formed.
In the specific case of air-jet looms, the pre-feeder lo also has the task of pre-measuring the length L, and this task is performed by counting the unwound turns of thread, for example by means of a photocell, since: EQU L=n.pi.D
where n is the number of unwound turns and D is the diameter of the fixed drum or reel of the feeder.
The unwinding of the thread is controlled by an electrically-controlled arrester, which, by means of a movable finger acting by contact engagement with the drum, stops the unwinding of the thread when the n.sup.th turn has been reached.
In conventional weft feeders, in view of the high speed of a modern air-jet loom, which can insert approximately 1500 meters of weft per minute, the intervention time of the arrester is extremely short and is typically comprised between 10 and 20 ms (milliseconds). It is evident that when the arrester intervenes, a peak T.sub.1 of the mechanical tension T of the thread occurs in the portion of thread downstream of said arrester, said tension varying in time t, as shown qualitatively in the diagram of the accompanying FIG. 1.
The peak T.sub.1 of the mechanical tension T must be damped appropriately, on penalty of weft thread breakage, which occurs more frequently as the count of said thread decreases. Various auxiliary damping means, interposed between the weft feeder and the loom, are currently used for this purpose. Typically, a conventional tension damping device is constituted by at least one set of three rollers, the intermediate roller being movable; the weft thread is passed between said rollers along a path that forms loops which during thread braking are straightened since the intermediate roller or rollers flex, thus allowing the thread to elongate and consequently damping said tension peak.
However, this known auxiliary damping system, and others, based for example on the flexing of elastic means, do not yield satisfactory results, mainly due to the inertia of the masses of the movable damping elements, which produces significant delays in the intervention of the system, thus limiting its effectiveness.
Said auxiliary damping systems are furthermore physically separated from the weft feeder and accordingly, in addition to requiring adequate installation spaces, they must be selected and adjusted both according to the characteristics of the feeder braking device and to the count of the thread being processed.